1. Technical Field
The present invention relates generally to motor speed detection and, more particularly, to sensing the speed of a spindle motor.
2. Description of Related Art
A block diagram of a conventional disk drive system is shown in FIG. 1. The disk drive system includes a disk drive microprocessor 8, a control logic circuit 10, a voltage fault detector circuit 12, a set of voice coil motor drivers 14, a voice coil motor 16, a head carriage 20, a set of read/write heads 21, a magnetic media 22, a spindle motor 24, and a set of spindle motor drivers 26. The disk drive system is operated in the following manner. A host computer 4 communicates through a controller 6 to exchange commands and/or data with the disk drive microprocessor 8. The disk drive microprocessor 8 forwards the commands and/or data to the control logic circuit 10 which responds by directing the spindle motor drivers 26 to rotate the spindle motor 24 and the magnetic media 22 at a substantially constant velocity. The voice coil motor 16 moves the read/write heads 21 over specific tracks on the magnetic media 22 in response to commands from the control logic circuit 10. Once the read/write heads 21 have stabilized over the appropriate tracks, they can read data from or write data to the magnetic media 22 in a conventional manner.
In conventional disk drive systems such as the one shown in FIG. 1, the magnetic media 22 rotates at a high velocity and the read/write heads 21 are narrowly spaced from the surface of the magnetic media 22. In this environment, the read/write heads 21 may easily make contact with the magnetic media 22 in what is known as a head crash which can have catastrophic results. Data may be permanently lost. In addition, the read/write heads 21 or the magnetic media 22 may be damaged by the head crash to such an extent that the entire disk drive system no longer functions. Therefore, virtually all modem disk drive systems are designed to minimize contact between the read/write heads 21 and the magnetic media 22. To this end, many disk drive systems park their read/write heads 21 when the disk drive system is powered down such that the read/write heads 21 land on a parking zone on the magnetic media 22 rather than on an area of the magnetic media 22 which stores data. A parking zone is an area of the magnetic media 22 where data is not stored and is typically selected to be over tracks closest to a center of the magnetic media 22. This selection minimizes wear on tracks in the magnetic media 22 where data is stored and increases the reliability of the disk drive system and the integrity of the data it stores. Additionally, many disk drive systems minimize wear of the read/write heads 21 by braking their spindle motors 24 as quickly as possible to stop rotation and minimize the amount of time the read/write heads 21 are dragging on the magnetic media 22.
When power is first supplied to the disk drive system from a supply voltage, the operation of the disk drive system is inhibited by the voltage fault detector circuit 12 until the supply voltage is considered to have stabilized. At that time, the voltage fault detector circuit 12 pulls a power on reset signal or POR signal on a line 11 to a low voltage. Conversely, the voltage fault detector circuit 12 pulls the POR signal to a high voltage when the supply voltage drops below a safe operating voltage. In this latter case the control logic circuit 10 is powered by a capacitor 9 until the read/write heads 21 can be retracted and the spindle motor 24 is braked. In the disk drive system shown in FIG. 1, there is a substantial risk of a head crash if the spindle motor 24 is braked before the read/write heads 21 have been retracted.
In the event of a sudden lack of supply voltage, a procedure is implemented to first park the reading and writing heads, and then to stop the spindle motor once the heads have reached the parked position.
In the absence of supply voltage, the spindle motor serves as a voltage generator, the voltage of which depends on its rotational speed and its electric constant.
FIG. 2 shows the block diagram of a typical power combination used in applications for hard disks. The power combination is used to properly drive the voice coil motor 16 and the spindle motor 24 by using the VCM driver 14 circuitry and the spindle motor driver 26 circuitry. The VCM driver 14 circuitry includes a driving circuit 11 and a power stage 13. The power stage 13 includes two pairs of transistors M7-M8, M9-M10 with respective pairs of diodes D7-D8, D9-D10 connected in parallel with each other between a supply voltage Vm and ground GND. The voice coil motor 16 is coupled to the shared terminals of the pairs of transistors M7-M8, M9-M10. The spindle motor driver 26 circuitry includes a driving circuit 15 and a power stage 17. The power stage 17 includes three pairs of transistors M1-M2, M3-M4, M5-M6 with respective pairs of diodes D1-D2, D3-D4, D5-D6 connected in parallel with each other between supply voltage Vm and ground GND. The spindle motor 24 is coupled to the shared terminals of the pairs of transistors M1-M2, M3-M4, M5-M6.
The external supply voltage VCV feeding the power part varies according to the type of application; in desktop applications (desktop PC) it is of 12V, whereas in mobile applications (laptop PC) it is of 5V.
The Iso-Fet circuit 36 is an internal element of the power combination that serves to insulate the internal supply line Vm from the external supply line VCV if the latter were to fail. The Iso-Fet circuit 36 includes a transistor connected to the voltage VCV and controlled by signal P; said signal P is adapted to shut down the transistor of the Iso-Fet circuit 36 when the voltage VCV is null, whereas it is adapted to keep the transistor on when the voltage VCV is positive.
When the voltage VCV fails, the backelectromotive force voltage of the rotating spindle motor, i.e., the BEMF (Backelectromotive Force), is rectified to keep the internal supply line Vm at a potential high enough to supply the section of the voice coil motor 16 for parking the heads. This BEMF can be used to charge the capacitor 9 (in FIG. 1) or charge some other capacitor or energy storage device coupled to the potential line for the voltage supply Vm.
The rectification of the backelectromotive force of the rotating spindle motor may be carried out by means of one of the following procedures, i.e., a passive rectification, a synchronous rectification of the BEMF of the spindle motor or a step up of the spindle motor.
The passive rectification implies a rectification of the BEMF of the spindle motor through the intrinsic diodes of the power stage 17 which is operated at high impedance.
The synchronous rectification of the BEMF of the spindle motor takes place in an active manner through the sequential power up of two MOSFET transistors of the power stage 17 in synchronicity with the phase of the three backelectromotive forces of the coils L1-L3 of spindle motor 24.
The rectification by means of the spindle motor step up implies that the power stage 12 is continuously switched from a tristate condition to a braking condition at a frequency higher than 16-20 kHz (out of the audible range), instead of being kept under the tristate condition. Thereby, when the power stage 17 is under the braking condition (with all the low side transistors being switched on or all the high side transistor being switched on as shown in FIG. 3A), the spindle motor 24 is under a short-circuit condition and therefore the three backelectromotive forces are able to generate a current in the motor. When the power stage 17 is driven in tristate as shown in FIG. 3B, the three motor currents generated during the braking step recirculate through the intrinsic diodes of the six transistors of power stage 17, thus loading the capacitance (for example, capacitor 9) connected between the line where there is the voltage Vm and ground GND, keeping it at a high enough potential so as to supply the power stage 13 and voice coil motor 16 with sufficient power for parking the reading and writing heads. Said parking procedure typically begins when rectifying the BEMF of the spindle motor 24.
In an alternate implementation, referred to as active step up, is shown in FIG. 3C and is provided in substitution for the tristate condition of FIG. 3B. In active step up, the three motor currents generated during the braking step recirculate instead through actively driven ones of the six transistors of power stage 17, thus loading the capacitance (for example, capacitor 9) connected between the line where there is the voltage Vm and ground GND, keeping it at a high enough potential so as to supply the power stage 13 and voice coil motor 16 with sufficient power for parking the reading and writing heads.
The parking procedure of the reading and writing heads may be commonly carried out either at constant voltage or constant speed.
In the case of constant-voltage parking, the voice coil motor 16 is driven by the stage 13 applying a constant voltage for a certain time period T1, preset with an appropriate polarity for moving the heads in the correct parking direction, or the voice coil motor 16 is driven by the power stage 13 applying a first constant voltage for a time period T1 and a second constant voltage higher than the first voltage for another time period T2.
In the case of constant-speed parking, the voice coil motor is driven so as to keep the speed of reading and writing heads controlled during the parking procedure. Various methods are known in the state of the art to keep under control the speed by which the voice coil motor takes the reading and writing heads to a parked position. This type of procedure ends when the heads reach the parking zone; the control circuit also includes means such as a circuit or device adapted to detect when the reading and writing heads reach the parked position.
Once the reading and writing heads reach the parked position, the spindle motor 24 may be stopped; this generally occurs by short-circuiting the coils of the spindle motor through the activation of the low side transistors or high side transistors of the power stage 17 in a triple half bridge configuration. This procedure is commonly called “dynamic brake”. Thereby, with the spindle motor being short-circuited, the BEMF thereof generates a braking current, which is a function of the amplitude of the generated BEMF, and therefore of the instant speed of the motor and the impedance of the motor coils.
In specific applications, such as the hard disks for high end applications, the rotational target speed is very high; the speeds for these applications typically range from 10,000 to 15,000 rpm. To achieve this speed, the impedance of the motor coils is made very low; in such a case, if the motor was placed under a short-circuit condition immediately after the parking procedure of the reading and writing heads, the current generated by the motor itself would have a value even higher than 5 or 6 amps.
In power combinations having the power stage with six transistors, such as the stage 17, such a current value very often exceeds the specification limits for the maximum current that may be driven by the power stage. For such a reason, due to reliability problems, at the end of the parking procedure the spindle motor braking may not be immediately activated through the power up of the low side transistors of the power stage 17, since the high current would damage the power stage 17. It is therefore required to wait for the speed of the spindle motor 24 to become lower than a predetermined speed Vf such that the generated BEMF then forces a lower current to the maximum level that may be controlled by the power stage when the spindle motor is short-circuited to activate its braking Therefore, in such a case, between the end of the heads parking step and the spindle braking activation there is a waiting time Ta in which it is controlled (by known means) when the speed of the spindle motor 24 becomes lower than the value Vf, so that the short-circuit of the coils thereof may be activated without the BEMF-generated current exceeding the specification limits of the power stage.
There would be an advantage if the speed of the spindle motor 24 could be determined for use at the very least in assisting with controlling the braking of the spindle motor after parking